“Rationalism”, like “socialism” and “impressionism”, is a very general term. Broadly speaking, it is the view that the innermost skeleton of the universe and the innermost skeleton of the human mind are one and the same. Reason – the “ratio” in rationalism – describes both what can exist and what can be thought. Several interesting consequences would follow from this view, if it were true. First, it would follow that if something is conceivable, then it is genuinely possible; and, similarly, if it is inconceivable, then it is impossible. Second, it would follow that nothing in the universe is (at least in principle) beyond our knowledge. And, third, it would follow that when the human mind discerns a logical relation between two ideas, the world must also exhibit a similar relation between the things corresponding to those ideas. In short, rationalism holds that the human mind holds within itself the key for understanding the structure of ultimate reality. For the deepest knowledge of reality, it is sufficient to look within.

In the past, rationalism was usually contrasted with “empiricism”, or the view that experience is the key to gaining knowledge of reality. That contrast has proven to be unreliable and misleading. It is not as if there was a group of card-carrying rationalists, who thought reason is everything and experience is nothing, facing off against a group of cardcarrying empiricists, who believed just the opposite. Reason and experience are both important to any plausible philosophy.